In this new application, we propose to explore the relationship between intracranial pressure (ICP) and intraocular pressure (IOP), two risk factors associated with glaucoma, a neurodegenerative disease of the retinal ganglion cells (RGCs) and optic nerve. The goal is to understand the impact of the opposing forces of ICP and IOP on the optic nerve head, and to identify changes in optic nerve and retinal structure and function following experimental alteration of ICP and IOP. A further goal is to determine the earliest changes in structure and function in direction-selective RGCs (DSGCs), a sub-type of RGCs believed to be particularly sensitive to changes in ICP and IOP. There are two specific aims: (1) to determine how the magnitude of the absolute ICP-IOP difference impacts optic nerve and RGC structure and function, and to test the hypothesis that maintenance of a stable absolute ICP-IOP difference mitigates the effects of either ICP or IOP elevation alone; and (2) to determine progressive changes in anatomy and response sensitivity in individual types of DSGCs with preference for each of the four cardinal directions of motion. We will employ a powerful and novel experimental tool that enables us to elevate the ICP of a living, awake mouse to a specified level for a prolonged period of time. In conjunction with established techniques to elevate IOP unilaterally, we are thus able, for the first time in active, unanesthetized mice, to adjust both ICP and IOP simultaneously. We will then use a series of anatomic, electrophysiologic and behavioral assessments over time and a range of ICP and IOP values to determine the relative contributions of ICP, IOP, and the absolute ICP-IOP difference to various phenotypes. Lastly, we will alter IOP in several transgenic mouse lines that mark ON-OFF DSGCs with motion preference for a specific cardinal direction (anterior, posterior, dorsal, or ventral) and assess dendritic arbors as well as functional changes in single cell loose patch or whole cell preparations. Our research will provide important insights into the mechanisms controlling cellular susceptibility to changes in ICP and IOP that can guide future translational diagnostic and therapeutic studies.